Eddington timescale: short, we can grow a small seed to large value quickly. By trapping radiation, timescales shorten

hot haloes collapse to supermassive stars -> BHs

1% of galaxies are quasars, BH mass is way lower than the total mass of the host: why are quasars short-lived? Because they are suicidal, self-regulating their growth, unbinding the gas that is feeding them.